Conventional ultraviolet nanosecond excimer lasers have been very successfully used for corneal refractive surgery such as photorefractive keratectomy (PRK), laser-assisted in situ keratomileusis (LASIK) and laser sub-epithelial keratomileusis (LASEK). By ablating corneal tissue through direct, one-photon absorption of ultraviolet light, these lasers are able to alter the curvature and thickness of corneas, ultimately altering their optical power.
The rapid development of femtosecond laser technology has provided an additional tool for corneal refractive surgery. In contrast to the photo-ablative ultraviolet lasers, femtosecond laser pulses in the near infrared or visible range can pass through transparent corneal tissue without significant one-photon absorption. Only when pulses are focused inside the cornea, is the intensity of the beam sufficient to cause nonlinear, typically, multi-photon absorption. Because the absorption is nonlinear, the laser-affected region tends to be highly localized, leaving the surrounding region unaffected, or minimally affected. See, Vogel A, Noack J, Huttman G, Paltauf G, Mechanisms of femtosecond laser nanosurgery of cells and tissues. Applied Physics B 2005, 81, 1015-47; Loesel F H, Niemz M H, Bille J F, Juhasz T, Laser-induced optical breakdown on hard and soft tissue and its dependence on the pulse duration: experiment and model. IEEE Journal of Quantum Electronics 1996, 32, 1717-22; and Giguere D, Olivie G, Vidal F, et al., Laser ablation threshold dependence on pulse duration for fused silica and corneal tissues: experiments and modeling, Journal of the Optical Society of America A 2007, 24, 1562-68.
In the past two decades, extensive experimental and theoretical work has been done to characterize laser-induced optical breakdown thresholds in different materials, including the cornea and the lens. Most of this work, however, centered on the use of continuous wave (CW) lasers or on single pulses from low-repetition-rate lasers in which thermal diffusion time is much shorter than the time interval between adjacent pulses. Thus, each pulse is responsible for a change in the material. Indeed, it has been established that for pulses longer than 10 ps, the optical breakdown threshold fluence scales as the square root of the pulse duration. To date, most femtosecond lasers used to cut corneas in clinical practice use microjoule (μJ) femtosecond laser pulses with a low-repetition-rate (Hz-kHz range) and spot diameters of more than 5 microns (μm). See, Kurtz R M, Horvath C, Liu H H, Krueger R R, Juhasz T, Lamellar refractive surgery with scanned intrastromal picosecond and femtosecond laser pulses in animal eyes, Journal of Refractive Surgery 1998, 14, 541-48; and Juhasz T, Loesel C, Horvath C, Kurtz R M, Mourou G, Corneal refractive surgery with femtosecond lasers, IEEE Journal of Quantum Electronics 1999, 5, 902-09.
This contrasts with the range of femtosecond laser parameters that have been established for biomedical applications. See, Loesel F H, Niemz M H, Bille J F, Juhasz T, Laser-induced optical breakdown on hard and soft tissue and its dependence on the pulse duration: experiment and model, IEEE Journal of Quantum Electronics 1996, 32, 1717-22. Compared with the low-repetition-rate femtosecond lasers with μJ or millijoule (mJ) pulse energies, high-repetition-rate (>1 MHz) femtosecond laser oscillators usually have pulse energies on the order of nanojoule (nJ). Such low-pulse-energy femtosecond lasers have been used for both micromachining and nanosurgery. See, König K, Krauss O, Riemann I, Intratissue surgery with 80 MHz nanojoule femtosecond laser pulses in the near infrared, Optics Express 2002, 10, 171-76.
While most femtosecond laser surgical procedures involve (by definition) some sort of disruption, either affecting membranes, organelles or other cellular components, they can be performed with such precision and selectivity so as not to kill the cells. Recently, research within our group on both silicone and non-silicone-based hydrogels, demonstrates that femtosecond micromachining works by inducing a significant change in refractive index of the materials without visible plasma luminescence or bubble formation, and without the generation of undesirable scattering or absorbing centers. See, U.S. patent application Ser. Nos. 11/745,746, filed May 8, 2007, and 11/948,298 filed Nov. 30, 2007. Our success with creating refractive structures in hydrogel materials led us to explore whether similar type structures could be created in ocular tissues.
There exists an ongoing need for ways to improve or correct vision. Changing the refractive index of ocular tissues, e.g., the corneal stromal layer or lens cortex, using a femtosecond laser, without tissue destruction or wound healing response would represent a major advance in the field of laser refractive correction or vision correction generally.